The present invention relates to a method and an apparatus employed to control the solidification of metal alloys, specifically Ni-base superalloys, in a plasma arc melting (PAM) and ingot casting operation.
For certain applications, particularly aerospace applications wherein nickel-base superalloy ingots are commonly employed, the ingot structure desirable is one free from structural imperfections. As used in this sense, the term imperfection includes but is not limited to laps, cold shuts, porosity, non-uniform grain size, and chemical segregation resulting in cracking or non-uniform mechanical properties. PAM processes provide a means to control the ingot structure and to minimize or eliminate imperfections by controlling heat input to the solidifying ingot. A further desired feature of such ingots is that they be free of oxide inclusions larger than the grain size of the finished component, as such inclusions adversely affect low cycle fatigue properties of the component. It is possible in some PAM processes to float oxide inclusions out of the molten metal prior to the inclusions entering the ingot mold with the molten metal.
Three basic methods are generally employed in PAM processes for producing metal alloys, namely drip melting, nonconsumable electrode melting and hearth melting. Generally, the end product formed in these processes is an ingot solidified from the molten metal in a casting mold. The drip melting process employs a feed stock electrode, which is melted using arcs, and the molten metal droplets fall on the upper surface of the ingot being cast. The nonconsumable electrode melting process employs feed stock which is introduced either directly into the molten metal in the casting mold or into a rotating skull crucible for melting and batch pouring onto the upper surface of the ingot. By comparison, the hearth-melting process employs a feedstock melted by plasma arcs wherein the molten metal is collected in a horizontal trough, or hearth, and is maintained as a liquid in the hearth by use of additional plasma arcs directed onto the surface of the hearth. This molten metal is then conveyed to a pour notch disposed over the ingot mold. It is known in the art in all of these processes that arcs may further be used to heat the upper surface of the metal in the mold to influence the solidification and cooling of the solidifying ingot. Proper cooling of the ingot is required in order to produce the desired alloy solidification structure and surface condition of the ingot.
Electron beam melting (EBM) processes are similar to PAM processes except that EBM processes utilize electron beams rather than plasma arcs and they are conducted under a vacuum instead of an inert gas. Methods for production of uniform fine grain ingots by the EBM drip process have previously been proposed. As an example, one approach employs a continuous casting method in which the upper surface temperature of the ingot is maintained below the solidus temperature of the alloy but still above a temperature which promotes metallurgical bonding between the molten metal droplets and the ingot surface. In this process, no means are employed for measuring the ingot surface temperature for use in controlling the drip rate and deposition pattern. Also, in this process, the application of heat input to the upper ingot surface has generally been regarded as undesirable, possibly because of the absence of means for taking direct surface temperature measurements for controlling drip rate and deposition pattern. The result of the use of temperatures at or below the alloy solidus is that the product is not a true ingot casting, but rather is an accumulation of metallurgically bonded solidified droplets which form pores and entrap contaminants, such as oxide inclusions, in the structure.
EBM hearth processes have heretofore also been proposed for the purpose of producing ingots with desired internal structures together with acceptable surface conditions, although the processes have not met with complete success. Such prior processes generally involved visual observation of the molten pool surface and temperature measurements of a discrete location or locations made by a two-color pyrometer, while an operator used such information in attempting to manually control the electron beam power and impingement pattern in order to produce a desired pool surface temperature with the object of yielding the desired ingot solidification structure. To date, this method of process monitoring has proved to be inadequate in attaining the required accuracy in controlling the beam power and impingement pattern to produce the desired ingot solidification structures.
In one previous approach to ingot casting by an EBM or PAM hearth process, the objective of the process has been to maintain the pool surface temperature at the center of the mold at a temperature slightly below the liquidus temperature of the alloy, while maintaining the temperature at the edges of the pool slightly above the alloy liquidus temperature. The former temperature was selected in order to create solid crystallites to act as `seeds` from which the ingot would solidify, and the latter temperature was selected in order to prevent cold shuts or laps from forming at the edges of the ingot. This process has the advantage that the central pool temperatures can be monitored visually because the formation of the crystallites provides a visual indication that the temperature is in fact below the alloy liquidus. As discussed above, however, visual observation and manual control of the pool surface temperature do not provide the degree of control accuracy which is required to produce ingots having the desired solidification structures.
This method has a further disadvantage in that the temperature gradients produced on the ingot pool surface in practicing this method also give rise to unacceptably rapid fluid convection in the pool. The rapid pool convection has the potential to take undesirable oxide inclusions from the surface and entrap them in the solidifying ingot. Additionally, the deliberate temperature gradient produced on the surface in this method results in a non-uniform microstructure in the solidified ingot. One further disadvantage which has been noted in association with this approach is that, when the pool temperature employed is below the liquidus, a very shallow ingot pool is evidenced, and the solidification structures produced are exceptionally sensitive to small changes in the energy applied in the form of beam or arc heating, making the process even more difficult to properly execute and control.
While the relatively narrow area of heat input characteristic of electron beams makes the precise spatial control of heat input possible, it also makes it difficult to maintain large areas of the molten metal surface at a uniform temperature. In addition, the high vacuum necessary for the use of electron beams restricts heat extraction and selectively vaporizes alloying elements. With an inert atmosphere in PAM processes, greater heat extraction is possible, perhaps creating a shallower pool to produce a satisfactory solidification structure. The inert gas atmosphere also reduces vaporization of alloying elements, making it easier to produce a desired ingot composition. By using arcs, PAM also has a broader heat input distribution than is characteristic of EBM, allowing easier maintenance of large areas at uniform temperatures.
It is therefore a principal object of the present invention to provide an apparatus for casting a molten metallic material in the form of an ingot wherein the solidification is accurately controlled to produce a predetermined desired solidification structure in the ingot.
It is another object of the present invention to employ an imaging radiometer in combination with a PAM hearth or drip melting apparatus, wherein the imaging radiometer is positioned to measure the upper molten pool surface temperature and provide an image related to temperature distribution across the surface.
It is another object of the present invention to provide a method for casting a molten metallic material in the form of an ingot, wherein the method includes accurately measuring and monitoring the upper molten pool surface temperature, and directing an arc at the upper molten pool surface to maintain a substantially uniform temperature across substantially the entire upper molten pool surface.
It is a further object of the present invention to provide a method for casting a molten metallic material in ingot form, wherein the upper molten pool surface temperature is measured by an imaging radiometer and an image related to temperature distribution across the surface is produced by the imaging radiometer, the image being employed to control the intensity and areas of impingement of arcs directed toward the upper molten pool surface in order to maintain the substantially uniform temperature across the molten pool surface.